Solar cells have been utilized as a driving energy source in various devices. Various proposals have been made for such solar cells. The proposed solar cells are of the constitution having a pn junction or a pin junction at the active portion. As the semiconductor material to constitute such junction, single crystal silicon or amorphous silicon is generally used.
As for the solar cells obtained by using a single-crystal silicon, although they are desirable since their photoelectric conversion efficiency is high, there are disadvantages that their production cost is relatively high and their mass production is difficult since it is extremely difficult to make them to be of a large area.
On the other hand, as for the solar cells obtained by using an amorphous silicon, although their photoelectric conversion efficiency is inferior to that of the solar cells obtained by using a single crystal silicon, there are advantages. In particular it is easy to make them so they have a large area, it is possible to mass-produce them, and their production cost is therefore low.
In recent years, solar cells which can be obtained by using a polycrystal silicon have been spotlighted in view that they can attain a high photoelectric conversion efficiency similar to that of the single crystal silicon solar cell and they can be provided at a reduced production cost similar to the production cost of the amorphous silicon solar cell. And not only various studies but also various proposals have been made of those polycrystal silicon solar cells.
As for the method of producing a polycrystal silicon solar cell, there is a proposal to use a plate-like material obtained by forming a lump of polycrystal silicon and slicing the lump formed. There is, however, a limit for the thickness of the plate-like material obtained in this case that is about 0.3 mm at the minimum, wherein it is extremely difficult to make the plate-like material to be of less than 0.3 mm in thickness. Because of this, the resulting polycrystal silicon solar cell unavoidably becomes such that it has an undesirably thick active region (that is, semiconductor layer) which is insufficient in material utilization, insufficient in photoelectric conversion efficiency and high in production cost.
In view of the above, various proposals have been made to form a polycrystalline silicon thin film by means of a thin film-forming technique such as a chemical vapor deposition process (CVD process). However, in any of those proposals, there are problems that the resulting film is of the order of about 10.sup.-2 micron meter in crystal grain size and a solar cell obtained by using such film is inferior to the solar cell obtained by means of the foregoing massive polycrystal silicon-slicing method in view of photoelectric conversion efficiency.
In order to solve these problems of the CVD process, there has been proposed an abnormal grain growing technique in which ions of an impurity of P or the like are implanted into a polycrystal silicon thin film formed by means of a CVD process to supersaturate the silicon thin film with said impurity and the resultant is annealed at elevated temperature to thereby enlarge the size of crystal grain about ten times over the film thickness (see, Yasuo Wada and Shigeru Nishimatsu, Journal of Electrochemical Society, Solid State Science and Technology, 125, 1499(1978)).
However, the film formed according to the proposed technique is of a remarkably high impurity concentration although it is satisfactory with respect to the crystal grain size. And a solar cell prepared by using this film is problematic in that light-induced carriers are often recombined and as a result, sufficient photocurrent is not provided.
Another attempt has been made in order to enlarge the crystal grain size of a polycrystalline silicon thin film formed by means of the CVD process has been made by subjecting the thin film to irradiation of laser beam to fuse the thin film, followed by recrystallization. However, this attempt is not practically usable since it is difficult to stably obtain a film of prescribed quality, and if a desirable film should be obtained, the resultant film unavoidably becomes costly.
The above situations are more or less the same in the case of compound semiconductors such as GaAs, ZnSe, etc.
Now, Japanese Unexamined Patent Publication Sho. 63-182872 discloses a process for producing a solar cell comprising disposing a different material on a surface of a substrate such that said different material forms a crystal nucleation surface, said different material having a nucleation density which is sufficiently greater than that of the constituent material of said surface of the substrate and being sufficiently minute to an extent that only a crystal nucleus capable of providing a single crystal can be grown after crystal growth; forming a crystal nucleus at the different material by way of deposition; growing a crystal based on the crystal nucleus to form a first conduction type semiconductor layer substantially comprised of a single crystal on said surface of the substrate; and a second conduction type semiconductor layer substantially comprised of a single crystal on said single crystal layer. This Japanese literature discloses that according to to this process, it is possible to obtain a thin solar cell with a large grain size which provides a good photoelectric conversion efficiency.
However, as for the process described in the above Japanese publication, there is a problem that because a metallic material is used as the nucleation surface on which the formation of a crystal nucleus is caused and the metallic material is used as an electrode, the constituent atoms of the metallic material are often diffused into a semiconductor layer formed to cause the formation of an alloy with the constituent atoms of the semiconductor layer, resulting in breaking the junction. Even in the case where such junction breakage does not occur, there is another problem that those diffused metallic atoms are greatly incorporated into the crystals or the crystal grain boundaries to increase the levels by which light-induced photocarriers are trapped.
Other than the above, U.S. Pat. No. 4,816,420 discloses a process of producing a solar cell by covering the surface of a single crystal silicon substrate with a mask member comprising an insulating film, lateral overgrowing single crystal silicons based on the single crystal silicons as seeds which are exposed through the spaced portions of insulating film, removing the overgrown silicon crystals from the single crystal silicon substrate, and transferring them onto a separate substrate.
However, there is a disadvantage as for this process that it is difficult to attain the production of a large area device because a single crystal silicon is used as the substrate. Particularly, in order to form a large area solar cell panel according to this process, it is necessary to provide a plurality of silicon crystals formed in the above-mentioned manner and to transfer those silicon crystals onto a separate large area substrate, and because of this, the production procedures becomes complicated and as a result, the resulting solar cell unavoidably becomes costly.